Collisions of highly vibrationally excited pyrazine (E vib = 37,900 cm(-1)) with HOD: state-resolved probing of strong and weak collisions.

This work presents state-resolved measurements of weak and strong collisions between HOD and highly vibrationally excited pyrazine (Evib = 37,900 cm(-1)). Transient IR absorption line profiles of HOD(000) rotational states that are populated in single collisions with pyrazine (Evib) are fit using double-Gaussian functions to extract Doppler-broadened line widths and energy transfer rates for appearance and depletion populations. We recently reported the use of this new approach to determine collision rates for energy transfer (Havey, D. K.; Liu, Q.; Li, Z. M.; Elioff, M.; Fang, M.; Neudel, J.; Mullin, A. S. J. Phys. Chem. A 2007, 111, 2458-2460). Here we present a full description of transient measurements of weak collisions, along with rate constants and the full energy transfer distribution function for the vibrational-to-rotation/translation (V-RT) pathway. The low- and high-J populations of scattered HOD(000) are characterized by a single rotational distribution with Trot = 430 +/- 50 K. The average translational energy of the scattered HOD(000) increases modestly with rotational energy suggesting an impulsive energy transfer mechanism. The energy gain distribution P(Delta E) for HOD(000) shows that approximately 99% of collisions have DeltaE values less than 3000 cm(-1). These data yield a lower limit to the collision rate that is approximately 70% larger than the calculated Lennard-Jones collision rate. These findings show that water is an efficient quencher due to a large collisional energy transfer cross section and not as a result of a large supercollision tail.